In today's radar systems there is a requirement of higher and higher linearity at more or less constant current consumption for an amplifier circuit.
If two signals are supplied to the two inputs of the amplifier, then the linearity of the amplifier may be defined as the ability of the amplifier to curb intermodulation products at the amplifier output per given signal power.
The difference between a useful signal and undesired intermodulation products can be written as IM3=2(IP3−Isignal) (expressed in dB). In this case IP3 is related to every tone, but it may also be related to the sum of powers on each tone, i.e. Isignal+3 or the vector sum of the tones, i.e. Isignal+6. IP3 is often given in dBm, but it may equally be put in relation to a current or a voltage. We will choose to relate IP3 to a current, since it is the current amplitude which influences the amplitude of the intermodulation products.
One common circuit for signal amplification is a cascode amplifier shown in FIG. 1.
The upper part of the circuit comprises a current follower with the transistors Q1 and Q2 whose function is to keep the output current iout at the same level even if its output is loaded by a high impedance which would lead to a high power consumption. Thus iout stays essentially the same even if R1 is increased.
The lower part of the circuit transforms an input voltage signal into an output signal current. Both stages use the same bias current ibias shown in the figure.
FIG. 7 shows the cascode amplifier together with the intermodulation products produced by the two stages of the amplifier. The diagrams to the left of the cascode amplifier display the signal amplitude expressed in dB as a function of frequency. By studying the diagrams in FIG. 7 it may be seen that the amplitude of the intermodulation products before and after the current follower stage remains the same. Hence the entire contribution to the increase of the amplitude for the intermodulation products comes from the transconductance stage of the cascode amplifier.
Examining the linearity of a cascode amplifier one notices that it is theoretically possible to decrease the bias current in the current follower and to increase the bias current through the transconductance stage comprising the transistors Q3 and Q4 without sacrificing linearity. However, this is difficult to do, since both stages share the same bias current ibias. Moreover, if the current saving due to the bias current reduction in the current follower stage and the subsequent current increase through the cascode amplifier should be of any use, the increased current should be put to use. Otherwise power consumption is reduced in the current follower stage only to be wasted in the transconductance stage.
However, the problem with this solution is that the transconductance stage cannot handle as high a signal current as the current follower at a given bias current. The reason for that is that an increase of the signal current through the transconductance stage would increase the amplitude of the intermodulation products in the output signal from the transconductance stage.
One other alternative may be to increase the bias current ibias in the entire cascode amplifier, i.e. in both the transconductance stage and the current follower stage. This solution however, wastes current and therefore power in the current follower where no amplification of the signal is achieved.
One other alternative may be increase the linearity of the amplifier circuit by adapting the transconductance stage to deliver a lower signal current than the current follower. This, however, would necessitate replacing the current follower by a current amplifier. Traditionally, current amplifiers are built with a transconductance amplifier, which in this case would not lead to a satisfactory solution.